1. Field of the Invention
The present invention generally relates to a servo method for a magnetoresistive (MR) head and to a hard disk system and, more particularly, to a servo method for positioning an MR head when information is reproduced by using the MR head and to a bard disk system provided with MR heads.
2. Description of the Related Art
As an information recording medium designed to record an enormous amount of information used in computers and the like, a hard disk drive (hereinafter, HDD) has been put to practical use. Such an HDD has been provided with a plurality of magnetic recording disks (hereinafter, disks). Reproduction and recording heads (hereinafter, magnetic heads) have been positioned on the desired tracks respectively determined on disks so that information has been recorded on disks or information has been reproduced from disks.
In these HDDs, rotary actuators using voice coil motors (hereinafter, VCMs) have been widely used for positioning magnetic heads at the prescribed positions over disks since they are light, operate at high speed, and are cheap to manufacture. This type of rotary actuator has a rotation axis outside the disk and turns the magnetic head arranged at its end onto the disk so that the magnetic head is positioned at a prescribed location on the disk. A servo system involves a sector servo system in which a pattern, a cylinder number, or a sector number necessary for position control each sector on the surface of each disk is recorded and which is extensively employed, especially for small HDDs.
With the recent increase in types of processing by computers and the like, the manufacturing of large-capacity HDDs has grown. To promote the manufacture of large-capacity HDDs, much work has gone into making compact reproduction and recording heads to be positioned over desired tracks of disks. Such a magnetic head for recording and reproducing information is often a magnetoresistive (MR) head.
This MR head is typically provided with a reproduction element and a recording element which are independent. The reproduction element is a MR element having magnetoresistance. The recording element is an inductive film head. The MR element is, as well known, an element having magnetoresistance (called MR) by which electrical resistance changes due to a magnetic field and magnetization, and can detect a magnetic field near the MR element. This MR element needs a bias magnetic field to improve the linearity of sensitivity and changing magnetic field upon detection of the magnetic field based on the change in a resistance value. As general methods for obtaining this bias magnetic field, such methods as shunt bias and soft bias are used.
In case where information is recorded or reproduced with a disk employing an MR head, a servo control to position the MR head, in other words, a servo control (called microjogging) for positioning the MR head at an optimum location is needed corresponding to the time at which reproduction and recording are done when the MR head is to be positioned above reproduction and recording media and the like.
For example, when the MR head is installed on a rotary actuator HDD, the MR element and recording element sometimes do not pass the same track location. Generally speaking, the track width of the recording element is larger than that of the reproduction element (MR element). Since the MR element is spaced a prescribed distance from the recording element, sometimes either the MR element or the recording element does not pass the same track because of the angle of skew and the difference in position between the MR element and the recording element in accordance with the turning of the rotary actuator, in prescribed different locations on the disk (for example, the location of a track near the inmost edge of the disk and the position of a track near the outmost edge of the disk) by turning the rotary actuator. Therefore, it is necessary to change the position of the magnetic head so that such processing as the recording and reproducing of information and the like with the disk can be done wherever the location may be on the disk.
Primary processing such as recording and reproducing information and the like with the disk includes data recording, data reproduction, and formatting. That is, according to a general sector servo system as shown in FIG. 9, the following areas are included on at least one track: servo areas 50 where servo information SERVO such as a cylinder number, a position control pattern, and the like is stored; ID areas 54 where sector information ID recorded at format control or thereafter; and data areas 56 where data information DATA is stored. Therefore, data recording processes for reproducing data information are done. For data reproduction, data reproduction processes for reproducing data information are done. For formatting, processes to record the sector information ID are done to record and reproduce data.
Consequently, in accordance with the sector servo system, it is essential that the sector information ID be rewritten at any time for the purpose of stopping the use of a defective sector, and that data information be reproduced immediately before processes without distinguishing between processes to record and reproduce data information. An example of processes to record and reproduce information and the like with the disk by using the MR head are described below.
As FIG. 9(A) shows, a magnetic head 20 for recording and reproducing information with a disk comprises a recording element 19 and a reproduction element 21 (MR element). Since data recording processes are important to stably obtain data information, data information DATA is recorded by the recording element 19 so that the center of the reproduction element 21 is located (following called on-tracking) in the center (location on the center line C.sub.L in FIG. 9) of the servo area 50 to stabilize the servo control. For data reproduction (FIG. 9(B)), data information DATA recorded is reproduced by moving the reproduction element 21 so that the center of the reproduction element 21 is near the center (location on the center line C.sub.M in FIG. 9) of the data area 56 where data information DATA is recorded. Formatting (FIG. 9(C)) records sector information ID in the ID area 54. With these processes, it is necessary that the recorded sector information ID should be reproduced in each data reproduction and recording process. Therefore, in formatting, the sector information ID is recorded in the ID area 54 so that the center (location on the center line C.sub.F in FIG. 9) of the recording element 19 is located in the center (near the intermediate location between the location on the center line C.sub.L in FIG. 9 and the location on the center line C.sub.M in FIG. 9) between the location of the reproduction element 21 positioned in data recording and the location of the reproduction element 21 in data reproduction. In this way, various types of information can be recorded and reproduced from the information recording disk.
To increase the capacity of HDDs as noted above, the track width is assumed decreased. However, in the case of HDDs in which the track width is to decrease, a problem arises in that the percentage of undetected or erroneously detected information (called the error rate) significantly increases (deteriorates) based on how much the MR head is dislocated from the center of a track (called off-track). Namely, while the track width has decreased, the magnetic head has also been miniaturized, so that the track width of the reproduction element as the MR element is also short. Thus, an error rate is affected sharply and sensitively by the amount of off-track which, in the most extreme case, causes tracks or sectors that cannot be reproduced to be produced.